Hi everyone!
This video will discuss the Qualitative Analysis of Anions, the experiment you will be performing
this week.
In the past two weeks, you have worked on the separating and identifying cations.
This week’s experiment focuses on anions.
In one respect, this experiment will be easier.
For some anions, you will not need to separate them before you can identify them.
This reduces the potential mistakes that can happen.
For other anions though, you will still need to separate them out from others, but the
steps are much simpler compared to the cation separation.
Another difference is that for the anions test, you will need to make two positive controls.
One positive control will just be the anion of interest, while the other will be a diluted
sample of the anion.
The reason we do this is because your unknown sample will contain anions whose concentrations
lie somewhere between these two positive control samples.
Knowing what the two extremes look like can prepare you for what to expect with the unknown
sample.
The last point I want to make is that the procedures
below always start with “mix 1 mL of your sample with …”. In the experiment, that
sample will be each of these: the two positive control or the unknown.
Alright, let’s get started with each of the tests.
The first test is for the carbonate anion and sodium carbonate is the positive control.
You will mix 1 mL of your sample with 1 mL of HCl.
You should see small bubbles floating to the top; this is the CO2 gas formed from the reaction
between proton and carbonate, an acid base reaction.
A picture of this effervescence is shown here.
The left one will be for your diluted sample and the right for your concentrated sample.
So in your unknown, if no effervescence is observed, then no carbonate is present.
The next test is for the sulfate ion, which will use sodium sulfate as the control.
You should mix your sample with HCl and then a few drops of barium chloride.
If sulfate is present, a white precipitate, as shown in the picture will appear.
The HCl is a catalyst that helps the reaction to occur, the actual reaction is just between
barium and sulfate ions.
The third test is for the phosphate ion, which uses sodium hydrogen phosphate as the control.
Here you will add nitric acid to your sample and then 1 mL of ammonium molybdate and stir
this mixture well to mix.
Then heat the mixture in a water bath, which will help to form a yellow precipitate of
ammonium phosphomolybdate to form if phosphate is originally present in the sample.
The precipitate appearance is shown in the picture for reference.
The reaction looks fairly complicated as written here, but it is really just a precipitation
reaction catalyzed by acid.
The next test is for the thiocyanate ion, which uses potassium thiocyanate as the control.
Here we will do a reaction you should be familiar with, the complex ion formation of iron thiocyanate
as shown here.
You will add acetic acid to your sample then two drops of the iron nitrate solution.
After stirring you should see a red blood solution of the complex ion, which forms under
acidic conditions.
Typically you will see the dark red solution as shown in the picture or the slightly lighter
one shown on the top right if your sample has thiocyanate.
There’s a potential problem with this test, which is if phosphate is present.
Recall that the previous test tells you if phosphate is present.
So if phosphate is present, then when you add your iron nitrate, instead of a red blood
solution, you will see a white precipitate instead.
The precipitate is iron (III) phosphate and it will interfere with your detection of thiocyanate.
You will have to remove all the phosphate first.
This is done by adding 2 drops of iron nitrate, mix to form the precipitate and centrifuging
and then removing the pellet.
To the supernatant you will add 2 more drops of iron nitrate to see if more precipitate
is formed.
If it is, then centrifuge and remove the pellet again.
Then keep repeating this step until addition of iron does not produce a precipitate.
At that point, check if the addition of iron gives you the blood red color.
If it does, you have thiocyanate.
If it doesn’t, then you don’t have it.
Note that a very light reddish color does not, does not indicate the presence of thiocyanate;
it’s just contamination.
This is a common false positive.
The next test is for the chloride ion using NaCl as your control.
In this case, you will add nitric acid and silver nitrate to your sample which will cause
the formation of a white curdy precipitate of AgCl as shown in the picture.
The reaction is shown here.
Now if your unknown contains thiocyanate, then this test will give you a false positive,
because silver and thiocyanate itself forms a white precipitate, which is the reaction
shown here.
Recall that you would’ve checked your unknown for the presence of thiocyanate already, so
if you do have thiocyanate, then you have to remove it first before you can test for
the presence of chloride.
To remove the thiocyanate, you have the follow the steps shown here.
Add 1 mL of your sample and 1 mL of 6 M nitric acid to a small beaker and then heat the solution
until the volume decreases by half.
The heating decomposes all the thiocyanate ion.
Once that happens, you can transfer the solution back into a test and add nitric acid with
3 drops of silver nitrate and stir to check for chloride ion.
If a white curdy precipitate forms, then chloride is present.
The last test is for the nitrate ion, using sodium nitrate as the positive control.
Here you will add 1 mL of your sample with nitric acid and a scopula of Al powder.
Then heat the mixture in a water bath.
While the mixture is heating, get a red litmus paper and wet it with water.
When the mixture is hot, put the litmus paper over the mouth of the test tube.
If nitrate is originally present, ammonia will be produced as a result of the reaction
shown here.
Since ammonia is a weak base and is a gas, it will evaporate to the top of the test tube
and react with the wet litmus and change it to blue.
Note that the litmus has to completely change to blue, and not just showing small specks
of blue.
The specks are due to splashes of sodium hydroxide in the test tube that occur as a result of
heating.
If your unknown has thiocyanate, the test won’t work, so you will have to remove the
thiocyanate first.
The removal occurs in two steps.
First you add copper sulfate to precipitate the thiocyanate as copper (II) thiocyanate,
which has a black color shown here.
You then centrifuge and discard the pellet containing the copper thiocyanate.
Since you have excess copper in the supernatant, you will need to remove it, by adding sodium
carbonate.
The copper and carbonate combine to form the copper (II) carbonate precipitate.
You centrifuge once more to remove the pellet.
Now you can test the supernatant for nitrate ion.
To do this, add a scoopula of Al powder, heat the test tube and check for the apperance
of ammonia using the damp red litmus paper.
The last points I want to make is about safety and waste disposal.
Just like the last two weeks, you want to have your standard PPE on.
All the waste should be disposed of in the chemical waste container.